It is known that a primary, active dipolar magnetic field propagated from a generating source can be used to induce another magnetic field in a conductive material remote from the primary magnetic field source. This induced magnetic field can then be sensed by a remote magnetic sensor for detecting the presence of the conductive material. The sensor itself typically operates by producing an electrical potential, referred to as induced electromotive force (emf), when in the presence of a magnetic field, which in this instance is intended to be the induced magnetic field propagated from the remote conductive material.
However, as would be expected, the primary dipolar magnetic field, which induced the magnetic field in the remote conductive material in the first place, will also induce emf in the magnetic sensor. Because the induced magnetic field propagated by the remote conductive material is typically much weaker, having a far smaller amplitude, than the relatively strong primary magnetic field, the primary field tends to interfere with or even entirely obscure the inductive effects of the smaller induced field on the magnetic sensor. It is therefore necessary to isolate the magnetic sensor from the primary magnetic field so that the sensor can detect the much weaker induced field without being blinded by the primary field. Previously, such isolation was achieved by physically separating the sensor from the primary field transmitter by as great a distance as practically possible. The larger the distance, the smaller the amplitude or far-field strength of the primary magnetic field and, accordingly, the lesser interference with the detection of the induced field. It should be appreciated that a prior art self-contained electromagnetic sensing apparatus, which includes both a primary magnetic field transmitter and a magnetic field sensor, is necessarily a very large apparatus if it is to function properly.
Such electromagnetic propagation and sensing apparatuses have been used in areas such as geophysical exploration to detect and delineate geological formations, mineral ore deposits, and groundwater resources, as well as to detect buried man-made objects, such as underground tank systems, unexploded ordnance, burial trenches, drums, landfills, and contaminant plumes. Other areas in which such apparatuses could be used include marine bathymetry, which is the determination of water depth in a body of water, and the detection of submarine wakes and other military detection applications.
A typical electromagnetic sensing apparatus used in these areas is a large torpedo-shaped airborne device, which is suspended above the ground or water by a helicopter or fixed-wing aircraft. Such an apparatus could also be manually carried above the ground by engineers or technicians, but typically these apparatuses are too large to be easily managed without some type of mechanical assistance. Due to the separation required between the magnetic field transmitter and the induced field sensor, such apparatuses are typically up to approximately 20 feet long and may weigh up to several hundred pounds. For easier transportability and operation, a smaller apparatus is needed. However, using previous technology, a smaller apparatus would operate much less effectively due to the above-discussed interference of the primary magnetic field.
An additional problem with these large prior art electromagnetic sensing apparatuses relates to the electromagnetic fields generated by the aircraft or other machinery needed to carry the apparatuses above. Engines, revolving propeller blades, turbines, etc. in aircraft create a plethora of electromagnetic disturbances that can wreak havoc on a highly sensitive magnetic sensor near the aircraft. A smaller sensing apparatus, however, would require the use of a much smaller aircraft, which would create far fewer electromagnetic disturbances, or could even be easily carried by a technician with none of the electromagnetic disturbances associated with machinery.
Therefore, in view of the above, there is a great need for a practical, compact electromagnetic sensing apparatus, which includes both a magnetic field transmitter and a sensitive magnetic field sensor, wherein the sensor can be located in close physical proximity to the magnetic field transmitter yet be magnetically isolated from the transmitter.